DESCRIPTION: The long-term objective of the proposed research is to establish the optimal dose of exercise for cardiovascular (CV) health. The specific aims are to test two hypotheses: (1) 8 months of physical training (PT) will enhance CV health of obese youths, (2) high intensity PT will be more effective than low intensity PT. Obese 13-16 year olds (n=180) will be randomized, within gender and ethnicity (black/white), into control, low intensity and high intensity groups. All groups will receive standard care (i.e. diet and exercise information), with the PT groups receiving controlled PT in addition. The primary outcome variable will be percent fat (measured with dual x-ray absorptiometry). Resting metabolism, diet and free-living physical activity (with doubly labeled water) will be measured. A comprehensive set of risk factors for coronary heart disease (CHD) will be assessed, including visceral adipose tissue (with magnetic resonance imaging) and left ventricular dimensions (with echocardiography). Self-concept and self-efficacy will be measured to gain insight into psychologic effects of the interventions. Heart rates will be kept in low (110-130 bpm) or high (160-180 bpm) intensity zones, with energy expended held constant. The hypotheses will be tested with data from all subjects, regardless of adherence to the prescribed regimens. The post-test will be on the morning following the last PT session to determine the impact of PT in obese youths who exercise regularly. To determine the longer-term effect of the interventions, a complete set of assessments will be made 8 months after termination of the interventions. This study will provide definitive data concerning the effect of exercise in obese youths. This information will contribute to the development of programs for treatment of adolescent obesity, with implications for primary prevention of CHD.